iOS - add disabledBackGesture (#872)

ios/RCCNavigationController.m

       [mergedStyle removeObjectForKey:@"statusBarHideWithNavBar"];
       [mergedStyle removeObjectForKey:@"autoAdjustScrollViewInsets"];
       [mergedStyle removeObjectForKey:@"statusBarTextColorSchemeSingleScreen"];
+      [mergedStyle removeObjectForKey:@"disabledBackGesture"];
       
       [mergedStyle addEntriesFromDictionary:navigatorStyle];
       navigatorStyle = mergedStyle;